Among image forming apparatuses for obtaining color images by the electro-photographic method such as a color copier or a color printer. There is an apparatus for transferring toner images of various colors formed on a photosensitive drum using an endless transfer belt available. As an apparatus using the transfer belt, there are an apparatus for feeding a sheet of paper between the transfer belt and the photosensitive drum and transferring a toner image on the photosensitive drum directly to the sheet of paper and an apparatus for primarily transferring the toner image on the photosensitive drum to the transfer belt and then secondarily transferring it to the sheet of paper. In the image forming apparatus using the transfer belt, for miniaturization of the apparatus or improvement of the maintainability, in recent years, there is an apparatus using a process unit for uniting the photosensitive drum, a main charger, a cleaner, and the transfer belt and mounting or demounting them from the main unit. The process unit drives the photosensitive drum and transfer belt by a motor installed in the main unit.
However, when driving the process unit having drive mechanisms such as the photosensitive drum and transfer belt by the motor installed in the main unit. There are possibilities that depending on the part precision or assembly precision of the process unit, the axial center of each drive mechanism may be shifted. When the axial centers of the photosensitive drum and transfer belt are shifted, the driving thereof becomes unstable and the image quality is lowered remarkably. Therefore, even if the axial centers are shifted, it is required to drive stably the photosensitive drum and transfer belt.
On the other hand, in Japanese Patent Application Publication No. 2001-5291, an apparatus in which a developing roller of a process unit is connected to a drive means of the main unit by the Oldham structure is disclosed.
However, the conventional Oldham structure can absorb a shift or a deviation of the axial center in the perpendicular direction to the shaft of the developing roller but cannot absorb a shift in the parallel direction with the axial center. Therefore, when the process unit is mounted in the apparatus body, there is a fear that the connection of the developing roller with the drive means of the main unit becomes unstable. When the connection with the drive means of the main unit becomes unstable like this, a transfer image is distorted and particularly on the transfer belt for superimposing toner images of a plurality of colors and obtaining a full-color image, color slipping is caused by vibration of the transfer belt, and the image quality may be lowered.
Therefore, an image forming apparatus and a process unit are desired such that when the process unit having a transfer belt is mounted on the apparatus body, regardless of a shift due to the manufacture precision of the process unit, the belt drive shaft of the transfer belt is surely connected to a motor via the Oldham structure, and the transfer belt is prevented from vibration, thus the image quality is improved.